U.S. patent number 4,922,253 [Application Number 07/292,983] was granted by the patent office on 1990-05-01 for high attenuation broadband high speed rf shutter and method of making same.
This patent grant is currently assigned to Westinghouse Electric Corp.. Invention is credited to Regis J. Betsch, Michael C. Driver, Harvey C. Nathanson.
United States Patent |
4,922,253 |
Nathanson , et al. |
May 1, 1990 |
High attenuation broadband high speed RF shutter and method of
making same
Abstract
A radio frequency transmitting shutter operable at low power and
high speed for use in the protection of a radar or electronic
warfare array comprising a multiplicity of individual conductive
beam members interconnected by electrostatic switches formed using
high yield, common photolithographic methods is disclosed. Further,
a photosensitive embodiment of this device which would allow
activation of the shutter wherever the light falls upon the shutter
during radar protection, is disclosed and claimed.
Inventors: |
Nathanson; Harvey C.
(Pittsburgh, PA), Driver; Michael C. (McKeesport, PA),
Betsch; Regis J. (Monroeville, PA) |
Assignee: |
Westinghouse Electric Corp.
(Pittsburgh, PA)
|
Family
ID: |
23127100 |
Appl.
No.: |
07/292,983 |
Filed: |
January 3, 1989 |
Current U.S.
Class: |
342/5; 200/181;
333/105; 333/262; 343/841 |
Current CPC
Class: |
H01H
59/0009 (20130101); H01P 1/10 (20130101); H01Q
15/002 (20130101) |
Current International
Class: |
H01H
59/00 (20060101); H01Q 15/00 (20060101); H01P
1/10 (20060101); H04B 001/38 (); H01P 001/10 () |
Field of
Search: |
;342/1,5
;343/909,872,841 ;200/181 ;333/101,105,262 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Petersen, Micromechanical Membrane Switches on Silicon, IBM J. Res.
Develop., vol. 23, No. 4, Jul. 1979, pp. 376-385..
|
Primary Examiner: Gensler; Paul
Attorney, Agent or Firm: Grunebach; G. S.
Claims
What is claimed is:
1. A high attenuation, broad band, high speed radio frequency
shutter, comprising:
a multiplicity of electrostatic switches said switches operable to
be actuated by a predetermined voltage into a closed position, said
switches further operable to allow the passage of a radio frequency
signal, striking said switches;
a switch support means, said electrostatic switches spaced a
predetermined distance from each other upon a surface of said
switch support means said predetermined distance being less than
the shortest wavelength of said radio frequency signal, where said
radio frequency strikes said switch support means, said radio
frequency signal being alternately blocked by said closed switches
and passed through said open switches; and
a multiplicity of conductive beam members mounted upon said surface
of said switch support means, said conductive beam members
positioned upon said surface of said switch support means at right
angles to each other, said conductive beam members interconnecting
said switches.
2. A high attenuation, broadband high speed radio frequency shutter
as in claim 1, wherein said switch support means further comprises
an insulating substrate.
3. A method of radar protection utilizing a high attenuation,
broadband, high speed radio frequency shutter, which method
comprises:
providing a multiplicity of electrostatic switches, said switches
interconnected by conductive beam members, said switches operable
to be actuated by a predetermined voltage into a closed position;
and
energizing said switches into said closed position thereby blocking
a radio frequency signal;
deenergizing said switches into an open position thereby
transmitting said radio frequency signal;
providing a switch support means, said electrostatic switches
spaced upon a surface of said switch support means at a
predetermined distance between said electrostatic switches, said
distance between said electrostatic switches being less than the
shortest wavelength of said radio frequency signal which is
alternately transmitted or blocked by said shutter.
4. An electrostatic switch, comprising:
a first insulating substrate layer;
a first high resistivity conductive layer, said
first high resistivity conductive layer deposited upon said first
insulating substrate layer;
a first support layer, said first support layer deposited upon said
first high resistivity conductive layer;
a first highly conductive patterned layer, said first highly
conductive layer deposited upon said first support layer;
a multiplicity of photolithographically displaced regions having a
circular configuration, said multiplicity of photolithographically
displaced regions positioned predetermined distance apart from each
other;
a multiplicity of planar, linear conductive members, said planar,
linear conductive members intersecting at right angles upon said
second, resistivity conductive layer; and
a multiplicity of electrical contact means positioned within said
photolithographically displaced regions, said electrical contact
means operable to pull into contact said planar; linear conductive
members.
5. A high attenuation, broadband, high speed radio frequency
shutter, comprising:
an insulating substrate;
a first high resistivity layer, said first high resistivity layer
deposited upon said insulating substrate;
a first insulating layer deposited upon said first high resistivity
layer;
a multiplicity of circular metal contacts, said circular metal
contacts spaced a predetermined distance apart upon said first
insulating layer;
a second high resistivity layer deposited upon said first
insulating layer;
a multiplicity of highly conductive strips deposited upon said
second high resistivity layer, said highly conductive strips
intersecting at right angles upon said second high resistivity
layer; and
a multiplicity of circular, photolithographically inscribed regions
surrounding said multiplicity of circular metal contacts, said
inscribed regions, being directly beneath portions of said
multiplicity of highly conductive strips, such that said strips
form cantilever arms over said contact and inscribed regions, said
cantilevered arms being operable to be electrostatically pulled
toward said contacts during said radio frequency shutter
operation.
6. A radar protection system, comprising:
a radio frequency transmitting and receiving means mounted behind a
conformal surface; and
a high attenuation, broadband, high speed, radio frequency shutter,
said high attenuation, broadband, high speed, radio frequency
shutter mounted upon said conformal surface directly over said
radio frequency transmitting and receiving means, said high
attenuation, broadband, high speed, radio frequency shutter further
comprising a multiplicity of electrostatic switches positioned upon
said conformal surface a predetermined distance apart, said
electrostatic switches operable to be interconnected by conductive
beam members, said distance being less than the shortest wavelength
of a radio frequency signal emitted or received by said radio
frequency transmitting and receiving means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to RF shutters and more particularly
to such shutters which include a multiplicity of individual
intersecting conductive beam members end connected with
electrostatic switches for low resistance and high open RF circuit
impedance.
2. Description of the Prior Art
There are many situations where it is advantageous to be able to
electrically vary the RF transparency of a physical surface. For
example, an RF energy source hidden behind a variable transparency
"shutter" array may be so hidden by maintaining the shutter array
in the OFF position up until the moment when the energy is to be
transmitted in, for example, a radar situation. The "shutter" as
viewed from outside would appear as a solid metal sheet, thereby
reflecting the incoming RF signals.
If the shutter is perfectly reflective for most of the cycle and
perfectly transmissive for the few microseconds necessary to
transmit and receive a radar signal, then the chances for discovery
of the source of this RF energy would be significantly reduced.
Desirable properties of a RF shutter array would include: (1)
negligible RF attenuation by the shutter array when it is in the ON
state, below a few tenths of a dB; (2) very high RF attenuation in
the OFF state of approximately -30 to -40 dB (with its attendant
highly reflective surface); and (3) broadband microwave operation
in both transmission and attenuation.
Another desirable property of a RF shutter array would be the
property of inexpensive fabrication in the microelectronic
format.
It is well known in the prior art to fabricate in a batch process
microelectronic switches.
The U.S. Pat. No. 3,539,705 issued to H. C. Nathanson et al., on
Nov. 10, 1970, entitled, "Microelectronic Conductor Configurations
and Method of Making the Same" describes small air gap metal
structures batch fabricated as part of a microelectronic component.
These spaced metal elements can be optionally closed by compression
bonding.
A second patent to H. C. Nathanson et al., issued Aug. 1, 1972,
U.S. Pat. No. 3,681,134, also entitled "Microelectronic Conductor
Configurations and Methods of Making The Same," is a divisional
patent of U.S. Pat. No. 3,539,705. This second patent claims the
method of fabrication for the device of the first patent,
specifically structures and methods of making such structures
involving spaced metal members in integrated circuits, such as for
cantilever beams in resonant gate transistors.
A United States patent to Heng et al., issued Mar. 12, 1974,
entitled "Microwave Stripline Circuits with Selectively Bondable
Micro-Sized Switches for In-Situ Tuning and Impedance Matching,"
U.S. Pat. No. 3,796,976, describes a microstrip line divided into a
multiplicity of short sections, each capacitatively coupled to its
neighbor by a cantilever switch. These novel switches were of a
predetermined length, (equal to fractions of a desired wavelength)
and are connected together to shift the phase of energy propagating
along their length thereby tuning and impedance matching the
microstrip circuits.
As can be seen in the above referenced patents, it is well known in
the prior art to fabricate compression bonded microelectronic
conductor switches. However, the use of these switches has been
limited to their obvious use as switching devices in electrical
circuits.
The problem to be solved therefore is the problem of the protection
and hiding of the existence of a radio frequency transmitting means
during its operation without interference with the transmitting
means effective operation while it is being protected.
SUMMARY OF THE INVENTION
This invention provides a high attenuation, broadband, high speed
RF shutter array operable to protect a RF transmitting means in
close tactical and combat environments.
The shutter array comprises a multiplicity of intersecting
conductive beam members end connected by easily fabricated
electrostatic switches having a predetermined spacing between each
electrostatic switch in a conformal application. Specifically, the
high attenuation broadband, high speed radio frequency shutter
array comprises a multiplicity of electrostatic switches where
these switches are operable to be actuated by a predetermined
voltage into a closed position. These electrostatic switches are
spaced upon the surface of a support means at a predetermined
distance. The period of spacing between the switches along the
conductive beam members is less than the shortest wavelength of a
radio frequency signal emitted by a radar, when the radar is
positioned behind the high speed radio frequency shutter.
The shutter array means would be operable, maintained and power
driven without detriment to the RF transmitting system. This
invention also encompasses a method of radar protection utilizing
the high speed shutter.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention reference may be had to
the preferred embodiment exemplary of the invention shown in the
accompanying drawings in which:
FIG. 1 is a top plan view of an individual electrostatic switch
having four conductive beam members, constructed in accordance with
this invention;
FIG. 1A is a cross-sectional view taken along line IA--IA of the
individual electrostatic switch as shown in FIG. 1;
FIG. 2 is a schematic representation shown in cross-section of an
embodiment of the optically driven individual electrostatic
switch;
FIG. 3 is an isometric, cross-sectional view taken along line
III--III in FIG. 4 of the tip of one of the conductive beam members
forming one segment of the individual electrostatic switches;
FIG. 4 is a plan view of the preferred embodiment of the shutter
array comprising a multiplicity of individual electrostatic
switches;
FIG. 5 is a schematic representation of an alternative embodiment
of this invention in a photo driven application;
FIG. 6 is a plan view of an array of electrostatic switches in use
in a smooth wing configuration for an electronic warfare array.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows a plan view of an individual electrostatic switch 2.
As can be seen in this plan view, four conductive beam members, 7',
7", 7'" and 7"" having cantilevered switches 21', 21", 21'" and
21"" in a photolithographically displaced region 15. Tips 11', 11",
11'" and 11"" of the conductive beam members 7', 7", 7'" and 7""
respectively are positioned above the contact region 17. The length
L.sub.c, of the cantilever switch 21"" including the tip 11"", and
the depth, d, of the conductive beam member 7"" are also shown.
One of the most important characteristics of an electrostatic
switch 2, as shown in FIG. 1 is its low metal-to -metal resistance.
If the electrostatic switch 2 were replaced by, for example, a PIN
diode, an ON resistance of 0.5 ohm (.OMEGA.) would require a
holding current of approximately I=KT/R, or about 0.25 amps per
switch. In this embodiment there are four/L.sub.c.sup.2 per node or
approximately 6400 switches per square centimeter. Obviously, a
holding current of 250 milliampere/switch would be prohibitive due
to extreme power consumption. It can be seen in FIG. 1 that the
depth of the strip conductive beam members 7', 7", 7'" and 7"" is
slightly decreased over the photolithographically eradicated area
15 where they have a depth of d.sub.O, as opposed to the depth d
for the overall conductor.
FIG. 1A is a cross-sectional view taken along line IA--IA of two
conductive beam members 7'" and 7' of the individual electrostatic
switch 2 which is more fully described in FIG. 1. In this view
taken along cross-section IA--IA, it is clearly shown that a
silicon substrate 13 has layered upon it, an oxide layer 12. A high
conductivity region 19 is embedded beneath the oxide layer 12
within the silicon substrate 13. A polysilicon support layer 14 is
layered upon the oxide layer 12. The conductive beam members 7' and
7'" which over hang the photolithographically inscribed region 15
are comprised of two layers; a layer of titanium-gold (Ti-Au) 6,
has a layer of gold (Au) 8 a top it. The bevelled conductive beam
members 7' and 7'" which are cut on an angle extend fully over the
inscribed region 15, tips 11' and 11" respectively extending over
the gold (Au) contact region 17.
For the electrostatic switch 2 as shown in FIGS. 1 and 1A, the
values of an exemplary switch, 2 for example in the 2 to 18 GHz
shutter are: ##EQU1##
As shown in FIG. 1A, one suggested embodiment for the device has L
equaling 250 microns (.mu.), d equal to 5 microns (.mu.), and W
equal to approximately 10 microns (.mu.). Note that with a L where,
L is the surface distance between any two conductive beam members,
of 250 microns (.mu.) or conductive beam member, the shutter
spacing would be approximately 60 times smaller than a wavelength
at 20 GHz which is approximately 1.5 cm. Thus, when the shutter
screen is in the "OFF" position we would expect that it would be
highly attenuating to the 20 GHz radiation providing that the
shutter resistance would be appropriately low. We note that the
resistance of a 5 micron (.mu.) thick, by 10 micron (.mu.) wide,
250 micron (.mu.)long segment of gold (Au) would be
R=(RHO.times.L)/wxt)-(2e.sup.-8 .times.250e.sup.-6)/(1e.sup.-6
.times.10e.sup.-6), where RHO is the coefficient of resistivity, L
is the length, W is the width, and t is the thickness of the
material or approximately one half of an ohm (0.5 .OMEGA.).
Further, in FIG. 1A the conductive beam members 7', 7", 7'" and 7""
rest upon an insulator surface from which a circular region has
been photolithographically removed so that these beam tips 11',
11", 11'", 11"" respectively freely extend in space over a circular
contact region 17 positioned below the insulator surface. All of
the metal conductive beam members 7'-7"" are connected electrically
together. These conductive beam members comprise high conductivity
layers 6 and 8. Beneath the high conductivity layers 6 and 8 are
two insulator layers 12 and 14.
Providing that the multiplicity of electrostatic switches make a
reproducible contact resistance on the order of 0.5 ohm (.OMEGA.)
or lower, the reflectivity of the OFF shutter would not be
significantly degraded from that of a highly attenuating 250 micron
(.mu.) pitch continuous metallic shutter.
In FIG. 2 is a cross section schematic representation of the
electrostatic switch 2. In this embodiment we have undercut the
polysilicon layer 14 under each of the individual conductive beam
members 7. A layer of insulating oxide 12 prevents short circuiting
over the highly resistive regions 19 and polysilicon 14, supporting
the individual conductive beam members 7 with tips 11 overhanging
the photolithographically etched region 15 on the surface of the
substrate 13 wherein the tips 11 overhang the gold (Au) contact 17
supported by the oxide 12. In the embodiment as shown in FIG. 2, a
voltage source 24 is shown directly interconnected to the
conductive beam member 7 wherein the voltage 24 is grounded, by
ground means 10' and the highly conductive region 19 is grounded as
is the overall back plated region which again is connected to
ground 10.
As also shown in FIG. 2, the addition of a bottom electrode 10 and
the use of a substrate of RF transparent photoconductive material
makes it possible to project patterns of light onto the the back or
front of the shutter as more clearly seen in FIG. 5 to effect the
closure of the multiplicity of individual switches wherever the
light falls upon them. Specifically if the photoconductor is made
conducting by illumination, the voltage on layer 19 is shorted to
the grounded layer 18, disabling the switching action leaving the
switches open. Thus, a photo-addressable spatially-selective RF
shutter could be produced. Such a device can also be used as a
Fourier Transform plane at radar frequencies. Therefore, the
structure shown in FIG. 2 would be yet another embodiment device 2.
In FIG. 2, if we fabricate an optically driven array, then the
silicon (Si) layer, 13 becomes a photoconductor capable of
photoconductivity. Further, the electrode 18 would be of a
phototransmissive material in the visible light range, comprised
of, for example indium-tin oxide InSnO.sub.3. The driving voltage
24 for the photoconductive application would be connected to the
high conductivity layer 19. It is assumed for reason of these
discussions that in an array all high conductivity layers 19 of the
individual switches would be electrically connected. In either the
photoconductive or non-photoconductive application the grounds of
all the switches would be electrically interconnected as well.
FIG. 4 is a plan view of a shutter array 40 comprising a
multiplicity of individual electrostatic switches 2. These switches
2 are made up of at least four individual vertical or horizontal
conductive beam members 7 formed upon an insulation layer 13 having
a high conductivity layer 19 deposited within it. The distance
between the conductive beam members 7 upon the surface of the
shutter, is length, L. The width of the conductive beam members 7,
shown as width W, is also a determining factor in the efficiency
and function of these devices.
As further shown in FIG. 4 an array of horizontal and vertical
electrically disconnected conductive members 7 is positioned on a
square grid wherein the spacing or distance, L is much less than
the shortest wavelength of the RF energy under consideration. The
tips, 11 of the conductive beam members 7 of normal length L can be
electrically connected by means of a plurality of electrostatic
switch assemblies 2 located at each junction of the four tips 11
such as illustrated in FIG. 1. Also, as shown in FIG. 1, the metal
conductive beam members 7 rest upon an insulator surface 13 from
which a circular region 15 has been photolithographically removed
so that these tips 11 are freely extended in space over a circular
contact region 17, a distance below the insulator surface 13. All
of the metal conductive beam members 7, for example, are connected
electrically together through a highly conducting layer of
polysilicon 14 layered above top insulation layer 13.
In FIG. 4, the shutter array 40, with its multiplicity of
independent electrostatic switches 2 are all closed simultaneously
by the application of a voltage on the order of, as previously
described, the voltage pull in or, VPI between the high resistivity
layer 19 and the device substrate insulation layer 13. In the
closed position, the multiplicity of switches 2 pulled in by the
electrostatic attraction between the individual horizontal or
vertical conductive beam members 7 and the surface of the substrate
layer 13 short the tips 11 together against the contact 17 in a low
impedance manner. This electrical shorting forms a conductive
shutter 40 of pitch length L, where L is much less than the RF
wavelength c/f, and where C is the speed of light and F is the
frequency under consideration. If the impedance of this shutter
array 40 is sufficiently low, RF energy will be markedly attenuated
in the transmission phase since the RF, E and H, vectors cannot
penetrate the tight spacing of the multiplicity of switches
comprising the screen. In contrast, when the shutter 40 is in the
disconnected or OFF position it provides tip-to-contact open switch
capacitance and the switches are independent of each other. Again,
each segment of the array would be small relative to the RF wave
length and each segment would follow the localized E and H field so
that the RF wave would suffer little or no attenuation as it passed
through the open circuit screen. If the periodicity or, L is a
small fraction of the wavelength at the highest frequency of
interest, the shutter array 40 should have both broadband
transmission and attenuation characteristics up to a frequency such
that the wavelength that comes in appreciable fraction of the
length L.
Further, the shutter array 40 as shown in FIG. 4 would require the
use of a multiplicity of electrostatic switches 2 operable as low
impedance shorts at negligible switching energy because a very low
capacitance would allow the ability of the electrostatic switch 2
to switch between the two states of open or ON and OFF in
microseconds. Note that a distance or spacing, L of approximately
250 microns, i.e., the spacing between the multiplicity of switches
along the conductive beam members being 250 microns apart, 6400
electrostatic switches 2 per square centimeter of a shutter 40
would be required. The ability to produce this many switches having
a high yield by photolithographic techniques all operable to
perform simultaneously in a reliable manner further constitutes the
novelty of this invention.
FIG. 3 is a cross-sectional view taken along line III--III as shown
in FIG. 4 of the tips 11 as found on the horizontal and vertical
conductive beam members 7. As shown in FIGS. 1 and 1A, the tips 11
overhang the gold contact 17. The length of this tip 11, which will
function as a cantilevered switch, may be referred to as L.sub.c,
or the cantilever switch length. The "d", is the depth of the
cantilever switch length L.sub.c. And, the distance between the
gold contact 17, and the cantilever switch tip 7, is referred to as
.delta..sub.0. In order to calculate the number of volts necessary
to actuate the individual tips 11 of the conductor 7 the voltage
pull-in (V.sub.PI), equations may be used.
For example, the pull-in volts V.sub.PI pull-in can be calculated
as follows: ##EQU2## where:
Y.about.10.sup.11 N/m.sup.2 (Gold)
Y/.delta..about.2.0.times.10.sup.3 meters/sec (gold), and
f.sub.R, mechanical resonant frequency of the conductor beam.
FIG. 5 is a schematic representation of the preferred embodiment of
an array 40 of electrostatic switches 2, in a photodriven
application. A source 30 of visible light, operable to emit photons
of energy 31 is positioned before a shadow mask 33. The shadow mask
33 has orifices 35 cut within it operable to define the visible
light 31 passing through the orifices 35. The shadow mask 33 allows
only a predetermined portion of the visible light 30 to pass
through the mask 33 to the lens 38. This lens 38 is then operable
to project the shadow mask defined light 31 onto an array 40 of
electrostatic switches 2 as optical pattern 42. The electrostatic
switches 2 more clearly shown in FIG. 4, form a mesh 41. These
electrostatic switches 2 will be selectively activated into a
"closed" position in those light exposed regions 43 of the optical
pattern 42 which receives the photons of visible light 31 from the
lens 38. The use of photodriven switches 2 in an array 40 for the
protection of a radar operating at narrow frequencies has now been
expanded by using a grid-shaped mask 33 to project a large grid
pattern and can now selectively actuate switches 2 thereby forming
a larger, broader frequency grid upon the array 40. The
electrostatic switch 2 as shown more clearly individually in FIG. 2
would be of a size of approximately 1/10 of an inch, further
comprising an embodiment having a ground plane transparent to
visible energy 31 such as, for an example InSnO.sub.3 (indium tin
oxide). This transparent substrate would permit rear projection of
the light pattern from the shadow mask 33 from within the airplane,
thereby actuating selectively rows of switches 2 upon the skin of
the aircraft and making a wide frequency range array.
FIG. 6 shows using a schematic representation of the use of a
shutter in a smooth wing configuration such as a cross-sectional
view of an airplane wing, operable to maintain low radar
cross-section while the shutter is in the OFF position. The shutter
array 40 is shown, as one example mounted conformally into the
curvature of the airplane wing 20 directly protecting the radar or
electronic warfare array 22. When in flight, the airplane wing 20
carrying the radar electronic warfare array 22 would be subjected
to incoming waves of RF energy 24 from outside radio frequency
energy sources "searching" for the radar. When the shutter array 40
is in the off position, the incoming waves of RF energy 24 which
strike the conformal surface of the airplane wing 20 from below
would be deflected as reflected RF energy 24'. There would be very
little scattering downward into outerspace of the reflected RF
energy waves 24'. It can be shown as in FIG. 6 that the total
energy to activate a 6000 electrostatic switch element, 100 volt
shutter array 40 is only approximately 10 microjoules, making the
switching energy required for a 0.1 meter square window of
approximately 6,400,000 switches only be 0.01 joule. With a window
frequency in the kilohertz range, only about 10 watts of power
would be necessary to power the shutter array 40, and this power
value could easily be reduced by a factor of 10 with judicious
geometry of the switches and biasing of the individual
electrostatic switches 2.
Additionally, it should be noted that during the ON time of the
shutter, as shown in FIG. 6, no power would be necessary to keep
the inherently infinite impedance electrostatic switches 2 closed
in direct contrast to the utilization of a PIN diode.
As can be seen in the previous drawings, specifically FIG. 4, FIG.
5 and FIG. 6, the RF, metallic mesh, high speed, broadband shutter
array 40 is operable to protect; using batch high yield fabricated
electrostatic switches 2 on a single shutter array 40, electronic
warfare or radar devices 22. The electrostatic switches 2 which
interconnect the conductive beam members would be fully functional
requiring extremely low power during the RF broadband spectrum and
the multiplicity of lower power switches in approximately 10 watts
per multisquare foot would allow a fast switching time in the area
of approximately 10 microseconds. This would be ideal for the
protection of RF energy emitters 22 from detection, in particular,
in high active warfare situations.
New variations may be made in the above described combination and
in different embodiments of this invention. They may be made
without departing from the spirit thereof. Therefore, it is
intended that all matter contained in the foregoing description and
the accompanying drawing shall be interpreted as illustrative and
thus not in a limiting sense.
* * * * *